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X-ray's Identity Becomes Crystal Clear

An unusual and unorthodox series of scientific discussions in a café led to Max von Laue's ingenious experiment that unmasked the true identity of X-rays.

In the years before the 1914–1918 War, Munich was one of the world's great hubs of scientific and artistic innovation. Painting, poetry and physics flourished in the capital of Bavaria – and a favourite meeting point of its protagonists was the coffeehouse at the Hofgarten, which was immortalised in T.S. Eliot's (Nobel Laureate 1948) poem "The Waste Land":

Summer surprised us, coming over the StarnbergerseeWith a shower of rain; we stopped in the colonnade,And went on in sunlight, into the Hofgarten,And drank coffee, and talked for an hour.

Drinking coffee and talking at the Hofgarten café provided respite for more than just poets. The café was also the setting for a series of informal discussions between a mixture of physicists from the nearby university, discussions that would play a crucial role in uncovering the true identity of X-rays.

Café Lessons

The central figure in these coffee meetings was Arnold Sommerfeld. Sommerfeld was appointed professor of theoretical physics in Munich in 1906, but he was trained as a mathematician. So to help improve his knowledge of experimental physics, he asked Abraham Joffe – the assistant of the discoverer of X-rays, Wilhelm Röntgen – for some coaching. Joffe proposed that he and Sommerfeld should meet every morning in the Hofgarten café to talk about experimental physics.

Sommerfeld, initially the eager student, was himself an exceptionally gifted teacher, and soon the informal discussions widened to include crystallographers and chemists. His Hofgarten café circle, together with a Wednesday evening symposium at the university, created an atmosphere of open and creative discussion that was unusual in academic circles at that time. Sommerfeld might not have received a Nobel Prize, but no less than seven alumni of the "Sommerfeld school" would become Laureates – Werner Heisenberg, Peter Debye, Isidor Rabi, Wolfgang Pauli, Linus Pauling, Hans Bethe, and, first of all, Max von Laue in 1914 for his discovery of the diffraction of X-rays by crystals.

What Is Light?

Max von Laue joined Sommerfeld's group as a private lecturer in 1909, and he was immediately struck by the atmosphere that was "saturated with questions for the nature of X-rays." Röntgen's discovery of X-rays in 1895 had created a major stir, not just among physicians and the public for its use as a medical imaging tool, but also among physicists trying to understand what these mysterious rays were made from.

At the time, physics was in the middle of one of its greatest transition periods. For over 250 years eminent physicists had argued whether light behaved like a wave or a particle. Five years after Röntgen's discovery Max Planck founded his quantum theory, which stated that energy is made up from individual particles, or "quanta", in a similar way that matter is made up of atoms. In 1905, Albert Einstein extended Planck's theory by showing that the photoelectric effect – the small current flow and release of heat that occurs when light shines on a metal surface – could only be interpreted to mean that light behaved like particles.

Over a decade after Einstein's explanation of the photoelectric effect, it would eventually become clear that light behaves both like a wave and a particle. But at the time of the Hofgarten café meetings nothing was certain about the nature of light, or indeed X-rays.

Röntgen had shown that X-rays behaved like light waves in some senses; for instance, the way in which they affected photographic film. Röntgen even proposed that X-rays behaved more like longitudinal waves, such as sound waves, than transverse waves, such as water ripples. However, Röntgen also found that X-rays could neither be deflected when they passed from one medium to the other – for instance, from air to water or air to glass – nor could they be reflected, which are two fundamental characteristics of light waves. X-rays were unlikely to be charged particles because not even strong magnetic fields could deflect their direction.

Einstein's discussions about the photoelectric effect influenced discussions about the nature of X-rays, as a letter to Sommerfeld in January 1910 shows. If X-rays were waves, writes Einstein, how should a piece of metal be able "to store pot-shards of X-ray waves like a housewife until it can equip one of its electron children with enough energy that it can travel through space with the intensity it is entitled to by its X-ray birth?"

Other evidence, however, appeared to contradict Einstein's views. Röntgen's initial suggestion that X-rays could be longitudinal waves had been abandoned in 1904 when Charles Barkla at Liverpool University had shown that X-rays could be polarized. Despite the objections of Einstein, polarization was a strong argument for a transverse wave. Further experimental evidence suggested that X-rays were a form of electromagnetic wave, with a wavelength at least ten thousand times shorter than visible light. With evidence slowly accumulating that X-rays could be waves, Sommerfeld wrote in a paper in Annalen der Physik in early 1912 that now the "proof of diffraction would be a sort of a keystone" for the wave theory of X-rays.

Crystal Conundrum

For all these discussions, it was in trying to solve another problem that von Laue unmasked the true identity of X-rays. Since his school days, von Laue had a long affection for optics, particularly the wave theory of light. So he jumped at the suggestion from Sommerfeld that he should write an article on wave optics for the Encyclopaedia of Mathematical Science, not only because it reunited him with his favourite subject, but also because it forced him to think about how crystals are organized.

In 1850, Auguste Bravais had suggested that crystals are made up from atoms arranged in regularly ordered, repeating geometric patterns in three dimensions – known because of their appearance as space-lattice structures. Many institutes in Munich University had mathematical models of these proposed space-lattice structures, mainly thanks to the enthusiastic support of the theory by the crystallographer Paul von Groth, but no one had yet proved that crystals have this structure. von Groth was another frequent participant of the Hofgarten café circle, and thanks to him von Laue quickly learned about crystal optics, and soon became known as a local specialist in the subject.

Two Birds with One Stone

One evening in February 1912, the physicist Peter Paul Ewald sought von Laue's advice about some difficulties he was having with his doctoral thesis on the behaviour of long electromagnetic waves in the hypothetical space lattices of crystals. von Laue couldn't answer Ewald's question, but his mind began to wander. Suddenly, a connection clicked in his mind. If diffraction and interference occurs when the wavelength of light is a similar size to the width of the slit of an optical grating, and if X-rays were indeed waves that have a wavelength at least ten thousand times shorter than visible light, then in theory the spaces between the atoms in a crystal might be just the right size to diffract X-rays. If all this were true, von Laue thought, a beam of X-rays passing through a crystal will be diffracted, forming a characteristic interference pattern of bright spots on a photographic plate.

Sommerfeld was sceptical when von Laue approached him with this idea. He doubted the experiment would work, and besides he needed his assistants for other assignments. Nevertheless, Sommerfeld was generous enough to give von Laue the go ahead to carry out the study. von Laue designed an experiment in which he placed a copper sulphate crystal between an X-ray tube and a photographic plate. His assistants, Walter Friedrich and Paul Knipping, carried out the experiment. After a few initial failures, they met with success on 23 April, 1912. X-rays passing through the crystal formed the pattern of bright spots that proved the hypothesis was correct.

In a single elegant experiment, von Laue had proven the wave-like nature of X-rays and the space-lattice structure of crystals at the same time. When he received the Nobel Prize for what the Committee said was his "epoch-making discovery", Max von Laue gratefully acknowledged Friedrich and Knipping for their roles in the discovery; and for him it went without saying that he shared his prize money with them.